0023466: database is not supported by PHP. Check that it has been compiled into your server.

Description

When upgrading from 2.6.0 -> 2.7.0, I now get a fatal error page when loading the main index.php:

APPLICATION ERROR #400
Database connection failed. Error received from database was #0: PHP Support for database is not enabled.

If I look at the admin/install.php page, I see:
Checking PHP support for database type BAD
database is not supported by PHP. Check that it has been compiled into your server.
Checking PHP support for MySQL driver BAD
mysql driver is deprecated as of PHP 5.5.0, and has been removed as of PHP 7.0.0. The driver is no longer supported by MantisBT, please use mysqli instead

Version 2.6.0 works perfectly in this environment. Version 2.7.0 does not.

Activities

Can I please suggest that the error message displayed be enhanced in two ways:
1) Tell you that you have the config option 'mysql' specified.
2) Test if mysqli is available, and if so - suggest changing mysql -> mysqli in the config file

or;
If mysql is specified, but mysqli is available, automatically switch to mysqli.

@vboctor I prefer to keep it as simple as possible.
There are already mappings that don't work,
e.g. function db_get_type does support types postgres and postgres7 whereas db_check_database_support does not support those types.
My plan was to have some more cleanup of the types after this PR.